The invention relates to a method for producing a hot strip by means of strip casting with material properties adjustable across the strip cross-section.
The hotly contested automobile market forces manufacturer to continuously look for solutions to reduce the fleet consumption while maintaining highest possible comfort and protection of occupant. Weight saving of all vehicle components plays hereby a crucial role as does a performance of the individual components at high static and dynamic loads during operation and in the event of a crash in order to promote the passive safety of the passengers.
Suppliers of raw material attempt to meet these demands by providing load-optimized metal sheets or strips of steel (e.g. tailor welded or tailor rolled blanks) which are optimized with respect to sheet thickness or made from materials of different strength to suit the load to be expected.
Such metal sheets or strip of steel have to meet comparably stringent requirements with respect to strength, stretch capability, toughness, energy absorption, and workability, for example through cold forming, welding and/or surface treatment.
The manufacture of load-optimized metal sheets of steel is disadvantageous because the welded sheet metal blanks require complex cutting and joining processes and exhibit sharp property gradients at the material transition.
DE 101 24 594 A1 discloses for example a method for producing a composite strip of steel. A directly cast ferritic core strip is hereby plated in accordance with the double roller process with an austenitic or high-alloyed ferritic plating strip.
The sharp jump of the properties of the composite material caused by plating is hereby disadvantageous because it complicates to suit the properties across the strip thickness to the requirement at hand. Furthermore, the properties cannot be varied across the strip width.
A method for producing hot strips of lightweight structural steel using a horizontal strip casting facility is known e.g. from the journal “steel research” 74 (2003), No. 11/12, page 724-731. Melt is fed in this method from a feed vessel via a casting channel onto a circulating casting belt of a horizontal strip casting facility. The fed melt solidifies when undergoing intense cooling to form a pre-strip with a thickness in the range between 6-20 mm. After thorough solidification, the pre-strip undergoes a hot rolling process.
This method is capable to produce in an ideal manner, e.g. lightweight structural steel having a high content of manganese that could otherwise be produced only in a difficult way when using conventional methods, like continuous casting.
The publication DE 199 18 581 A1 discloses the casting of thin strips of carbon steels, whereby the strip strength is enhanced by subjecting the strip to a carburizing or nitriding treatment. This can occur directly after casting or after casting followed by cold rolling and annealing.
Heretofore, this known strip casting method is however not adequate to produce hot strips of steel which have load-optimized material properties across the strip cross-section.